The back of the Futuramic lamphead has a built-in exposure guide. Set this so the guide number for the film you are using lines up with the point of the ready-light opening. For medium-speed films such as Plus-X the guide number is 90. For high-speed films such as Tri-X, the number is 135. For Ultra-high speed such as Ilford HP5, (400 speed), the guide number is 190. For Kodachrome, the number is 35. Fred.